Fluid pressure sealing system for processing oven



June 10, 1969 c. A. WINDSOR` 3,448,969 I FLUID PRESSURE SEALING SYSTEMFOR PROCESSING OVEN FLUID PRESSURE SEALINGVSYSTEM FOR PROCESSING OVEN yFiled Jan. a. 1968 June 10, 1969 A. wlNlvnsoR Sheet e M 5.. 0i WA. .Mn,f UAHHN-M/ ,C

United States Patent O 3,448,969 FLUID PRESSURE SEALING SYSTEM FORPROCESSING OVEN Charles A. Windsor, Plymouth, Mich., assignor toMichigan Oven Company, Romulus, Mich., a corporation of Michigan FiledJan. 8, 1968, Ser. No. 696,392 Int. Cl. F27b 9/28; F26b 13/00, 21/00U.S. Cl. 263-3 6 Claims ABSTRACT OF THE DISCLOSURE Background of theinvention There are a number of heating processes in which the bestconfiguration for the oven or furnace is a vertical tower havingopenings at the top and at the bottom through which the work enters andleaves the oven. This is particularly true with respect to processesinvolving la continuous web, strip, filament or wire, referred togenerically throughout this specification as ya strand There are also anumber of processes of similar nature in which the work is passedthrough a vertical oven, either on a continuous conveyor or by gravity,which require a similar oven construction.

In a vertical oven with openings at the top and the bottom there is anatural draft effect tending to pull cold air into the bottom of theoven and to discharge hot air out of the top of the oven. This naturaldraft effect wastes heat and makes it difficult or impossible tomaintain adequate control over the operating temperatures in the oven.Moreover, the discharge of hot air from the top of the oven heats thearea above the oven excessively unless some effective means of sealingthe top opening in the oven is provided. Since the continuous strand orother work must pass through the opening at the top of the oven the sealat that opening must be achieved by air pressure or air current.

The most commonly used means for sealing the top opening in a verticalpass-through oven entail the use of either a pressure box or a venturinozzle. For a pressure box seal, an enclosure is built around the topopening of the oven, this enclosure having a top opening of its ownthrough which the work passes. The pressure enclosure or box isconnected to a blower and air is blown into the box on either side ofthe work in a carefully distributed manner. This air escapes from thetop of the pressure box through the opening that passes the work. Theopening at the top of the pressure box and the volume of air blown intothe box are adjusted in relation to each other so that the pressure dropof the air leaving the top opening in the box is essentially equal tothe stack or draft effect within the oven. Thus, when the pressuresbelow and above the top opening in the oven are equal there is noappreciable flow of air either into or out of the top opening in theoven. Pressure box seals of this general kind are shown in Patent No.3,183,605 of Argue et al. and in Patent No. 3,186,698 to Thome.

In a venturi seal, the recirculating air in the top portion of the ovenis blown downwardly into the oven through a wide nozzle constructed toproduce a low pressure efice fect above the point where therecirculating gases enter the oven. The combination of the downwardvelocity of the recirculating gases and the venturi effect of the nozzletends to overcome the draft effect within the oven. Under certainconditions, in fact, the venturi nozzle may actually pull cold air inthrough the top opening of the oven. This may also happen in a pressurebox seal if the pressure within the box becomes excessive in relation tothe pressure within the oven. Generally speaking, Venturi seals lareused primarily in convection heating ovens whereas pressure box sealsare used on both radiant heating ovens and convection ovens.

Either the pressure box seal or the venturi seal can be adjustedmanually to afford a good working seal at any specific operatingtemperature and air flow conditions within the oven. But if theoperating temperature is changed, as may frequently be required, theseal is no longer fully effective. Thus, when operating condi- Itionswithin the oven are modified, there may be a substantial loss of heatedair through the top opening of the oven or an excessive amount of cold-air may be pulled into the oven through that opening. In eitherinstance the efficiency and effectiveness of oven operation arematerially reduced.

Summary of the invention It is an object of the present invention,therefore, to

. provide a new and improved fluid pressure seal means,

which may include either a pressure box seal or a venturi seal, thatautomatically compensates for changes in operating temperature, Within aprocessing oven of the kind having a top opening through which the workpasses either entering or leaving the oven.

It is an additional object of the invention to provide a new andimproved self-compensating fluid pressure seal system for the topopening of a processing oven that is inexpensive in construction yeteffective and durable in operation, and which requires a minimum ofadditional apparatus incorporated into the oven structure.

Accordingly, the invention is directed to a fluid pressure sealingsystem for a processing oven of the kind comprising a housing, means inthe oven housing defining an elongated vertical treating chamber havingupper and lower openings permitting passage of a strand of material ofindefinite length through the chamber, and heating means for heating thechamber, the normal operating temperature of the oven being sufiicientto produce an appreciable draft or stack effect tending 4to impel heatedair or other gas outwardly through the upper opening of the chamber. Thefluid pressure sealing system comprises fluid pressure seal means,including a blower, for maintaining a minimal pressure differentialacross the upper opening, thus preventing substantial movement of gasthrough that opening. Athermal sensor is located within the upperopening of theA oven housing and detects temperature changes within-that opening. Control means comprising a variable speed drive for theblower is coupled to the thermal sensor and operates to vary the speedof the blower in response to changes of temperature within the upperopening of the oven housing, maintaining the aforesaid pressuredifferential within a limited range and preventing excessive movement ofgas through the upper opening, either into or out of the oven chamber.

Brief description of the drawings FIG. 2 is a side elevation view, inpartial cross-section,

of the oven and pressure seal of FIG. 1;

FIG. 3 is a front elevation view, partly in cross-section, of a verticalprocessing oven, incorporating another em- Ibodiment of the presentinvention; and

FIG. 4 is a side elevation view, partly in cross-section, of the oven ofFIG. 3.

Description ofthe preferred embodiments FIGS. 1 and 2 illustrate avertical processing oven 10 comprising an insulated housing includingside walls 11 and 12 and front and back walls 13 and 14. The bottom wall15 of the oven housing includes an opening 16 through which a continuousstrand workpiece 17 enters the oven. The top wall 18 of the oven housingincludes a similar opening 19 through which the continuous str-and 17leaves the oven. `In the particular construction illustrated in FIGS. land 2, strand 17 is a continuous strip of foil bearing a coating thatrequires heating for the purpose of drying and curing the coating.However, a continuous web or strip of any other material requiring ovenprocessing could be treated in a process oven of similar conguration.

In oven 10, the direction or movement of the foil strip 17 through thecentral heating chamber 21 of the oven is generally indicated by thearrows 22. That is, the strip enters the oven chamber through the bottomopening 16 and passes directly up through the oven and out the topopening 19. A cooler 23 is shown mounted above oven and is used to coolthe coated foil 17 after it leaves the oven. 'It should be noted,however, that the foi] could equally well move downwardly through theoven entering at the upper opening 19 and exiting at the lower opening16 in the oven housing, in which case cooler 23 would be located belowthe oven. Where a two-stage oven is employed, it is known practice topass the strip or other work strand through the oven moving in an upwarddirection through one section of the oven and in ,a downward directionthrough the other; both types of construction present the draft effectproblem to which the present invention is directed.

Oven 10 (FIGS. 1 'and 2) is a convection oven. An input blower 24 isconnected to an entrance chamber 25 containing a burner 26 that blowsheated air through appropriate ducts 27 and 28 into the bottom of thevertical treating chamber 21 in the oven housing. The heated air movesupwardly through chamber 21 as indicated by the arrows 31 and isexhausted from the upper part of the treating chamber 21 through a pairof suitable exhaust ducts 32. An appropriate blower (not shown) may beemployed to exhaust the heated air from the oven chamber and to impelthe heated air to a suitable exhaust stack. On the other hand, if a twostage oven is employed, the heated air exhautsed from oven 10 may tbeused as the input to asecond stage in the oven system.

A pressure enclosure or box 41 is mounted on the top of oven 10intermediate the top wall 18 of the oven and the bottom of cooler 23.Pressure box 41 is disposed in encompassing relation to the upperopening 18 in the oven housing and is itself provided with upper andlower openings through which -the continuous strand 17 passes as itrnoves lfrom the oven to the cooler. Pressure box 41 is connected by iaduct 42 to a blower 43. The pressure box is provided with appropriateinternal batlies 44 for balanced distribution of air entering thepressure box from duct 42, maintaining the air pressure on both sides ofstrand 17 approximately equal. Pressure box 41 may be provided with'adjustable members 46 for adjusting the elective width of the upperopening 45.

A thermal sensor 48, which may be a conventional thermocouple ormercury-expansion element, is mounted within the -upper opening 18 ofthe housing of oven 10. Thermal sensor 48 is electrically connected toan adjustable control circuit 49 that is in turn electrically connectedto a variable speed drive 51 that constitutes the drive for blo-wer 43.Control circuit 49 and variable speed drive 51 thus comprise a controlmeans for varying the speed of blower 43 in response to changes oftemperature within the lupper opening 18 of the oven housing.

The particular forms of thermal sensor, control circuit, and variablespeed drive selected for units 48, 49 and 51 in oven 10 are notcritical. As a typical example, the variable speed drive may comprise anelectrically motor, using a transistor amplifier, of the kindmanufactured by Dynamatic Division, Eaton, Yale & Towne, Inc., Kenosha,Wis., in which the amplitude of a D.C. current supplied to a clutch coilis varied to regulate the torque transmitted from the motor to theblower, thereby regulating the blower speed. Variable speed drives ofthis type are usually provided with some means, such as thepotentiometer 52, for adjusting the normal operating speed of the drive.The thermal control unit 49 may comprise a temperature control such asthe Model LFP of The Partlow Corporation, Utica, N.Y., a modulatingcontrol that includes a potentiometer 53, in this instance connected tothe potentiometer 52 in the amplifier circuit of the eddy current clutchcontrol. But it will be recognized that many other specific forms ofspeed control apparatus suitable for controlling blower 43 in accordancewith the temperature of sensor 48 'are available and can be utilized.

In the operation of oven 10, room air or other appropriate processinggas is blown into the oven by means of blower 24, and is heated torequired temperature by burner 26. The heated air enters the oven at thebottom at ducts 27 and 28. The heated air passes upwardly through theoven (arrows 31) and is exhausted from the oven `by ducts 32 and exhaustplenum 34. Blower 43, on the other hand, forces air under pressurethrough duct 42 and into pressure box 41. The drive for blower 43 andthe opening in the top of the pressure box are adjusted relative to eachother to establish operating conditions such that the pressure at thetop opening 45 of the pressure box is equal to the pressure at thebottom opening of the pressure box, which is directly connected to thetop opening .19 in the oven. With these conditions established, thenormal draft or stack etect within the treating chamber 21 in oven 10 isbalanced `out and none of the heated air from the oven escapes throughopening 19 in the top of the oven. Furthermore, with accurate balancing,none of the relatively cold air from pressure box 41 is forceddownwardly into the oven.

With the operating temperature and pressures reasonably stabilized, Ithecoated foil or other strand 17 is passed through the oven to effect thedesired drying, curing, or other processing for the toil.

Subsequently, it may prove necessary or desirable to modify thetemperature within the heating chamber 21 or to change the volume ofheated air or other gas owing through the treating chamber of the oven.A change of this sort may be made deliberately or may result from avariation in operating conditions elsewhere in the overall system of theprocess of which the oven 10 forms a part. A substantial change of thisnature creates an unbalanced condition insofar as the pressure at theoven top opening 19 in relation to the pressure at the pressure box topopening 45 is concerned.

A temperature drop within the treating chamber 21 of oven 10 reduces thedraft effect in the oven and produces a lowering of the pressure at theupper opening 19 of the oven. If this occurs, there is a tendency forcold air to be drawn into the oven through top opening 19. Anysubstantial movement of cold air in this direction, however, produces atemperature change that is detected by thermal sensor 48. The change intemperature is signalled to control circuit 49 which in turn actuatesthe variable speed drive S1 and reduces the speed of blower 43. Byreducing the speed of blower 43, the ilow of air through duct 42 isreduced and a corresponding reduction is effected in the air pressurewithin pressure box 41. A balanced condition is achieved when thepressure at openings 19 and 45 is again equalized and the systemcontinues in stable operation until there is a subsequent change intemperature or air flow, of a substantial nature, within the oven.

Of course, the reverse operation occurs if the oven temperature ismaterially increased. A-n increase in the oven temperature produces acorresponding increase in the draft elfect within treating chamber 21and produces a higher pressure at the oven opening 19 than at thepressure box opening 45. The resulting outward ow of heated air or othergas from the furnace, through opening 19, produces a temperature changethat is detected by thermal sensor 48. This temperature change producesan electrical signal that is supp-lied to control circuit 49 and causesthe control circuit to increase the speed of variable speed drive y51.The llow of air through duct 42 from blower 43I is increased until abalanced pressure condition is again achieved, aifording continuingstable operation.

It is thus seen that the control means comprising circuit 49 andvariable speed drive 51, Iactuated by thermal sensor 48, varies thespeed of blower 43 in response to any change of temperature within theupper opening 19 of the oven. In this manner, the pressure differentialacross the upper opening 19 of the oven is maintained at a minimum,being held in balanced condition with respect to the pressure at the topopening 45 of the pressure box. By maintaining the pressure differentialacross opening 19 within a limited range, excessive movement of air orother gas through upper opening 19, either into or out of treatingchamber 21, is prevented.

FIGS. 3 and 4 illustrate a processing oven 100 that is different inconstruction from oven (FIGS. 1 and 2) and that incorporates a fluidpressure sealing system constructed in accordance with the invention andmodified to meet the requirements of the different oven construction.Oven 100 is a two-zone oven employed for drying and curing the insulatorcoating on insulated wire, the first or bottomI zone being designated byreference numeral 101 and the upper zone being indicated by referencenumeral 102. Oven 100 includes a housing comprising front and rear walls111 and 112 and side walls 113 and 114 The bottom wall 115 of the ovenhousing is provided with an opening 116 for admitting theinsulator-coated wire 117 into the oven. The top wall 118 of the ovenhousing is provided with an opening 119 through which wire 117 leavesthe oven.` In processing, wire 117 moves continuously through the ovenfrom the bottom to the top in the direction indicated by the arrows 122.

The lower section or zone 101 of oven 100 includes a heat source `andfresh air inlet 124 that feeds into supply plenum 125. A bottom zonecirculating fan or blower 126 draws heated air from plenum 125 and blowsthe heated air into a duct 127. A series of pressure drop baiiies 128may be mounted within duct 127. Duct 127 terminates in an outlet 129that feeds into a lower treating chamber 121 through which the coatedwire or other strand workpiece 117 moves. Additional heat may beprovided in the lower treating chamber I121 by radiant panels 130mounted upon the interior walls of the treating chamber.

The upper zone 102 of oven 100 includes a fresh air inlet 141 locatedwithin an interior portion 142 of the oven above an upper zone heatsource 143. A catalytic combustion element 144 is located below heatsource 143- between the heat source and an exhaust plenum y145. Exhaustplenum 145 is provided with `an exhaust duct 146 that is connected to anexhaust blower 147.

Exhaust plenum 145 takes its air or other gas froml the same opening inzone 102 as does an upper zone recirculating fan or blower .148 whichleads to a distribution duct 149. A series of pressure drop baflies 151may be located within duct 149. Duct 149 extends up the back wall 112 inthe upper zone of the oven andv across the top of the oven, terminatingin a venturi nozzle section y152 that discharges into an upper treatingchamber 153 through which stread 117 moves during operation of the oven.Chamber 153 extends downwardly through the front of the oven and into awidened portion 154 that is in direct communication with the upper end155 of the lower treating chamber 12. The two treating chambers are thusconnected to Ian opening 156 that leads into a duct 157 that extendsback to the oven section 142 containing the fresh air supply inlet 141and heater 143.

A thermal sensor 161 is located Within an upper opening 119 of oven 100and is electrically connected to a control circuit 162. Control circuit162, in turn, is electrically connected to a variable speed blower drive163 that constitutes the drive for blower148l in the upper zone of ovenIn operation of furnace 100, heated fresh air is drawn into the lowerzone of the oven from supply and heater 124 into plenum 125 and theninto duct 127 by means of blower 126. The movement of the heated airwithin the lower zone of the oven is indicated by arrows 131. From duct127, the heated air ows outwardly of opening 129 into the bottom portionof the lower treating chamber 121. The heated air impinges upon strand117 as the strand is passed through treating chamber 121 and is drawnolf through opening 156 in the upper zone of the furnace into duct 157.

Air from duct 157, and additional fresh air from duct 141 as required,passes through the portion 142 in the upper zone of the oven and isheated by the catalytic preheater 143. The heated air, which containssubstantial quantities of solvent and other volatile materials from theinsulator coating on strand 117, then passes through the catalyticelement 144, combustion within the catalytic element increasing theavailable heat in the furnace. A part of this air may 'be drawn offthrough exhaust duct 146 -by blower 147. But most of the heated air isexhausted from plenum by blower 148 and blown into duct 149.

In duct 149, the movement of the air, as indicated by the arrows 171, isin an upward direction, the heated air subsequently being divertedacross the top of the oven land outwardly of duct 149 through venturinozzle i152. The heated air moves downwardly through treating chamberI153, in contact with the upwardly moving strand 117, and is exhaustedback into duct 157 through opening 156.

In oven 100, it is the venturi nozzle =152 that is employed to providean effective seal for the upper opening 119 at the top of the furnace.That is, the recirculating air for the top zone of the ovenis |blowndolwntwardly into the treating chamber 153 in the top zone of the ovenand produces a low pressure effect above the outlet of the nozzle 152.The combination of the downward velo city of the recirculating ages andthe venturi effect of the nozzle overcome the stack or draft elfect inthe oven. When the temperature and rate of flow of the gases areproperly adjusted, there is little or noy ow of air into or out of theoven through opening 119.

But oven 1100, like oven 10, may require adjustment of the temperatureof those gases in order to maintain effective processing with respect tothe coating on strand '117. When any substantial adjustment is made, inthis regard, the balance of the venturi pressure drop with respect tothe stack effect of the oven is disturbed. For example, if thetemperature within either zone of the oven is increased materially, theresulting increase in draft or stack elfect may produce a substantialouttow of heated air and other gases .from the opening 119. By the sametoken, la material reductionI in temperature in either zone of the ovenmay cause cold fresh air to be drawn in through opening 1'19. In eithercase, effective processing within the oven may be prevented.

Variations in the draft elfect within oven 100 are balanced andcompensated by the operation of thermal sensor 161 in conjunction withthe control means comprsing control circuit 162 and variable speed drive163. Thus, if temperature conditions within oven v100 change to reducethe draft effect within the oven, the resulting inrush of air throughopening119 produces an appreciable drop in the temperature within thatopening and this temperature change is detected by thermal sensor '161.The thermal change is signalled to control circuit :162 which actuatesvariable speed drive 163 to decrease the speed of blower 148 and thusreduce the pressure drop effected by venturi nozzle 152. Restoration ofa balanced condition is established when the temperature in opening 119returns to its normal level.

The reverse action occurs when thermal conditions within oven 100produce an increased stack effect, forcing hot gases from the oven outthrough opening 119 and thus increasing the temperature to which thermalsensor 161 is subjected. Under these circumstances, the thermal sensoractuates control means 162 and 163 to increase the speed of blower 148,increasing the pressure drop effect of venturi nozzle I152 and againestablishing equilibrium in the oven system with respect to top opening119. It is thus seen that the fluid pressure sealing system of FIGS. 3and 4 operates in the same basic manner as that of FIGS. 1 and 2 tomaintain any pressure differential at the top opening of the oven at aminimum and thus maintain an effective seal at that top opening despitesubstantial changes in thermal conditions within the oven.

In the control means of either embodiment of the invention, as describedabove, it is not desirable to ernploy an overly-sensitive controlcircuit. That is, if the normal temperature at the top opening of theoven is 500 F. it is not desirable to have the lower speed changed inresponse to small incremental temperature differences, such asdifferences of 5 to 10. Rather, there should be considerable inertia inthe control system, with respect to temperature changes, in order toavoid excessive hunting in the operation of the control. Preferably,upper tand lower threshold temperatures are established, in the controlcircuit, with the blower speed being maintained at la set constant speedfor any temperature between the two thresholds. A deviation from therange intermediate the two thresholds then causes a change in blowerspeed as described above. This arrangement has the advantage of avoidingexcessive hunting and also tends to prevent changes in operation of theblower due to short-term thermal variations.

Hence, while preferred embodiments of the invention have been describedand illustrated, it is to be understood that they are capable ofvariation and modification.

\I claim:

1. In a processing oven of the kind comprising a housing, means in theoven housing defining an elongated vertical treating chamber havingupper tand lower openings permitting passage of -a strand of material ofindefinite length through the chamber, and heating means for heating thechamber, the normal operating temperature of the oven being sufficientto produce Ian appreciable draft effect tending to impel gas outwardlyof the charnber through its upper opening, a fluid pressure sealingsystem for that upper opening comprising:

fluid pressure seal means, including a blower, for maintaining a minimalpressure differential across said upper opening to prevent substantialmovement of gas therethrough',

a thermal sensor located within said upper opening to detect temperaturechanges therein;

and control means, comprising a variable speed drive for said blower,coupled to said thermal sensor, for varying the speed of the blower inresponse to changes of temperature rwithin said upper opening andthereby maintain said pressure differential within a limited range andprevent excessive movement of gas, through said upper opening, eitherinto 0r out of said chamber.

2. A fluid pressure sealing system for a processing oven, according toclaim 1, in which said fluid pressure seal means comprises Ia pressurebox disposed in encompassing relation to said upper opening of saidoven, and said blower is a separate blower employed to Iblow lair intosaid pressure box.

3. A fluid pressure sealing system lfor a processing oven, according toclaim 2, in which said control means increases the speed of saidseparate blower above a given normal speed only irf response todetection by said thermal sensor of a temperature above an upperthreshold level and decreases the speed of said separate blolwer belowsaid normal speed only in response to detection by said thermal sensorof a temperature below a lower threshold level, said upper and lowerthreshold levels being appreciably different from each other.

4. A fluid pressure sealing system rfor a processing oven, according toclaim 1, in which said oven is a convection o-ven in which heated gasesare introduced into the upper portion of said treating chamber through aventuri nozzle located adjacent said upper opening and are supplied tosaid venturi nozzle by said blower and in which said control meansdecreases the speed of said blo'wer whene-ver said thermal sensor`detects a substantial drop in temperature below a lower threshold leveland increases the speed of said blower ywhenever said thermal sensordetects a temperature above an upper threshold level.

5. A fluid pressure sealing system for a processing oven, according toclaim 4, in which said blower is -an integral part of said oven andconstitutes a recirculation blower for continuously recirculating heatedgases lwithin said oven.

6. A fluid pressure sealing system for a processing oven, according toclaim 4, in which said control means increases the speed of said blowerabove a given normal speed only in response to detection by said thermalsensor of a temperature above an upper threshold level and decreases thespeed of said blower below said normal speed only in response to'detection by said thermal sensor of a temperature below a lowerthreshold level, said upper and lower threshold levels being appreciablydifferent from each other.

References Cited UNITED STATES PATENTS 2,991,989 7/ 1961 Martin 263-33,183,605 5/1965 Argue et al. 263-3 X 3,209,467 10/ 1965 Taylor 34-1553,351,329 11/1967 Thomas 263-3 JOHN I. CAMBY, Primary Examiner.

U.S. Cl. X.R. 34--242

